If the ESC (F11) key on a VT220 or VT240 terminal is generating a string of
characters such as ESC [ 2 3 ~, the terminal is in the wrong mode.  To fix it:

1.  Press the F3 (Set-Up) key.  This will display the "Set-Up Directory" on
    the screen.

2.  Use the arrow keys to move the cursor to the "General" field.  Press the
    Enter key (at the right-hand side of the keyboard).  This will display the
    "General Set-Up" on the screen.

3.  Use the arrow keys to move the cursor to the "VT200 Mode, n Bit Controls"
    field (where 'n' is 7 or 8).  Pressing the Enter key will cycle this field
    through its possible values.  Press the Enter key several times, until the
    field reads "VT100 Mode".

4.  Press the F3 (Set-Up) key again.  this removes the "General Set-Up" from
    the screen and returns you to normal operation.

Note that if you were using Emacs when you performed this procedure, it is a
good idea to refresh the screen by typing Ctrl-L.